Jhonn, and then Jack posted about this guy they know who both agree has some kind of spiritual parasite on him. Both expressed a mingled compassion and disgust, and I can fully understand that. I didn't watch the videos because I'm trying not to pollute my brain with any more bull shit.
But running into people with parasites is pretty common in the occult world. Working with spirits tends to give you spirit sight, the ability to see when people are being influenced by spirits. Once you've started seeing them in your Work, you'll see them everywhere if you pay attention.
While doing my Goety Work, I ran into a lot of Nepheshim, the parts of the human soul that are supposed to die with the body, but often find ways to linger through offerings made to ancestors, finding geomagnetic or geoelectrical sources of food, or by attaching themselves to the living and triggering the amygdala (panic attacks, depression, and multiple forms of anxiety disorders) to release the emotive food they live on. These spirits are the most common entities I've seen, and while they are individually rather weak as a rule (with some exceptions), gathering up a legion or so isn't that difficult, and they're the original "Will Work For Food" spirits. With enough of these guys aimed and working for you in creative ways, there's potentially no limit to what you could get to happen.*
But for most who end up running into schools of them on the astral, you become host to a spiritual parasite. Regular banishings and visualizations of golden or white light helps, even the stuff you read in Fortune's Psychic Self Defense or whatever it was are effective against these guys.
But sometimes, as in the case Jack and Jhonn's acquaintance, people can pick up things that are ... bigger, stronger, worser. Some are like beefier versions of a Nephesh, something that's been around a while and has learned not only to feed and survive, but to thrive on people. Other times people can pick up a spirit of place with ill intent. Not all the Genius Loci are friendly. Heracles had to fight several Genii Loci in his Tasks, for example.
And of course, there are demons who will demonize humans given a chance.
I like the term "Larvae" to refer to these entities. Gordon posted a neat ritual from ancient Rome that was performed yearly and involved scattering black beans around the house while performing an oration. I suspect the beans absorbed the spirits, and were then planted. This returned any spirits that were haunting people to the cycle of reincarnation that Plato and Hermeticists loosely followed. They believed that those who did not attain their potential in the form of a man were demoted to the form of a woman**, and then to an animal, and then to a vegetable, and finally to a mineral state of being.
Jason's first book on Protection and Reversal Magic offered a lot of useful techniques for getting rid of these kinds of things, but when they're attached to one's self personally, it can be difficult to accept you've got one. Or a Legion in some people's cases. They can be pretty convincing that they're innocuous, and even helpful entities. They can offer techniques that really work, and they can appear as many things.
How can you tell if you're being possessed, obsessed, or demonized? How do you know if an entity, or paradigm shift is really what it appears to be, and not the product of something looking for a free lunch, or up to some plot that requires you doing a certain thing at a certain time?
You can drive yourself crazy worrying about it, and even if you didn't have an infestation of spiritual Larvae eating your essence, the anxiety generated by worrying about whether your HGA is secretly SATAN can attract a pool of 'em in no time.
If you're worried about it, the first thing to recognize is that you have a problem. Either there is a Larva, and that's the problem, or you just think there's a Larva, and that's the problem. Either one can cause mischief in your life. Fortunately, the solution for both is the same: Exorcise. If you do have an infestation, a good Exorcism will clean you right out. If you don't, it won't hurt. Incorporating some form of Exorcise in your regularly scheduled Work is useful.
The Stele of Jeu, that is, the Headless Rite makes for a formidable Exorcism. I generally stick to the Betz version, with a few modifications based on my experiences. I substitute the name of my HGA where appropriate, and "Agathosdaimon" for "Good Messenger of God." I also change "deliver him, NN, from the daimon that restrains him" to "deliver me, [my name/Hermetic Title] from any daimon that restrains me". It takes very few minutes, and can be worked in at any time. I prefer Sunday nights or Wednesdays, or both as necessary.
One of the more important things to realize as a magician, however, is that you always have a spiritual ally fighting off these things on your behalf. It comes with being born, and wears the names Agathosdaimon, Genius, and Holy Guardian Angel. Its name is hidden in your Natal Chart, and regardless of the level of conscious communion with this entity you have attained, it is still working on your behalf all the time. It's not proof against stupidity. We can conjure up a dark passenger through ignorance or just corrupt intentions, and that can limit the effectiveness of our Spiritual Ally in protecting us.
Learning to trust this spirit is essential, and if after performing an exorcism with its assistance you find yourself losing a spirit or source of peace and power in your magical Work, don't panic. Learn to accept it, don't conjure it back into your life, no matter how beneficial it may appear. The HGA will explain why it removed entities if you ask it. It will show you in vivid detail the tendrils of disgusting ooze the thing had immersed in your sphere.
Performing regular exorcism and trusting the results will build up your relationship with the HGA, and will open more avenues to reclaiming your divine race and value than it will close. Trusting your HGA also provides a level of peace in your approach that itself becomes a natural barrier to attracting Larva.
* Of course, that's necromancy, and tends to leave the magician who works with these fellows infested if they slip and forget to cleanse their spheres well after working with them. If you get into necromancy, and then find yourself all nihilistic, depressed, or anxious, you've got a problem you need to address. You've picked up a Larva. My Work with Bune granted me a degree of native exorcist, and directing them away with a statement carried enough weight to keep me clean, mostly, but regular Exorcism involving something like the powers of the Agathosdaimon or Michael the Archangel is a best practice mages do well to adopt.
** I disagree with the idea that being a woman is less than being a man, for the record. I think that's a cultural oppression thing that reflects the politics of the time rather than the spiritual truth. It's actually a step up from older beliefs, though. Before Plato's Republic talked about Soul Mates, theologists and spiritual cosmologists doubted women had souls at all.